Method to improve the life span of a heating element of a molybdenium disilicide at lower temperatures

ABSTRACT

A method of increasing the length of life of heating elements that consist essentially of molybdenum-silicide and alloys of this basic material, when said elements are operated at a low temperature, such as at a temperature ranging from 400-600° C. The invention is characterized in that the heating element material contains Mo(Si 1-x Al x ) 2 , where said material is caused to contain sufficient aluminium to substantially prevent the formation of pest.

[0001] The present invention relates to a method of increasing the useful length of heating elements at low temperatures, and more specifically the useful life of elements of the molybdenum-silicide and the molybdenum-tungsten silicide type, including different alloys of these basic materials. Elements of this nature are produced by applicants in a number of forms.

[0002] When such elements are operated at relatively low temperatures, for example at temperatures around 400-550° C., no protective scale of silicon dioxide, so-called glass skin, will form, as is the case when operating the elements at high temperatures. Instead, the elements are subjected to so-called pest attack, meaning that a non-protective layer of MoO₃ and SiO₂ forms on the surfaces of the elements. This mixture is porous and readily disintegrates, resulting in a significant shortening of the useful life of the elements.

[0003] However, there are applications in which such elements are, nevertheless, the best alternative. One example in this regard is found in the heating of LPCVD (Low Pressure Chemical Vapour Deposition) chambers in the manufacture of electronic circuits.

[0004] One way of improving the low temperature properties of such heating elements is to pre-oxidise the elements at a temperature of about 1500° C. or higher, so as to form SiO₂ scale. Such scale will slow down the formation of pest. Pest occurs as a result of MoSi₂ and O₂ forming MoO₃ and SiO₂. This oxide mixture is relatively porous and hence will afford no protection against continued oxidation.

[0005] Such methods, however, do not afford satisfactory protection against pest formation.

[0006] Another method is described in Swedish Patent Specification No. 0001846-5. This method involves causing the water content of the atmosphere surrounding the elements to be kept low when said elements are operated. This method provides satisfactory protection against pest formation.

[0007] The method according to the present invention significantly increases the length of life of such elements.

[0008] The present invention thus relates to a method of increasing the useful length of life of heating elements that consist essentially of molybdenum-silicide and alloys of this basic material, when the elements are operated at a low temperature, such as a temperature ranging from 400-600° C. inclusive, wherein the method is characterised in that the heating element material contains Mo(Si_(1-x)Al_(x))₂, where this material is caused to contain sufficient aluminium to significantly prevent pest formation.

[0009] The present invention is intended to increase the length of life of heating elements that consist essentially of molybdenum-silicide and alloys of this basic material, when the elements are operated at a low temperature, such as a temperature ranging from 400-600° C. inclusive. Examples of such furnaces have been mentioned above.

[0010] By molybdenum-silicide and alloys of this basic material is meant that the molybdenum-silicide may, for instance, be alloyed with a minor quantity of tungsten.

[0011] According to the present invention, the resistance material of the heating element includes Mo(Si_(1-x)Al_(x))₂, which is caused to contain sufficient aluminium to significantly prevent pest formation.

[0012] It has been found, very surprisingly, that no pest or only a very small amount of pest, is formed when such material is operated in said temperature range.

[0013] This is due to the formation of Al₂O₃ on the surfaces of the element, thereby reducing pest formation or causing such formation to disappear.

[0014] According to one preferred embodiment of the invention, x is caused to lie in the range of 0.2-0.6. When practising this embodiment of the invention, very little pest is formed in comparison with conventional elements used in the temperature range concerned.

[0015] According to a highly preferred embodiment of the invention, x is caused to lie in the range of 0.45-0.50. No pest is formed when practising this embodiment. Applicants have run a test in which one such element was held at a temperature of 450° C. for twelve months without pest forming. This was a very surprising result

[0016] According to one preferred embodiment of the invention, the heating element material was caused to contain up to 40 vol. % Al₂O₃. The aluminium oxide constitutes a mechanical element-stabilising phase. 

1. A method of increasing the length of life of heating elements that consist essentially of molybdenum-silicide and alloys of this basic material, when said elements are operated at a low temperature, such as at a temperature ranging from 400-600° C., characterised in that the heating element material contains Mo(Si_(1-x)Al_(x))₂, where said material is caused to contain sufficient aluminium to substantially prevent the formation of pest.
 2. A method according to claim 1, characterised in that x lies in the range of 0.2-0.6.
 3. A method according to claim 1 or 2, characterised in that x lies in the range of 0.40-0.50.
 4. A method according to claim 1, 2 or 3, characterised in that the heating element material contains up to 40 vol. % Al₂O₃. 